Stop bath acidity indicator



JZ uumumlnmmmm lili CRABTREE LD D. RUSSELL Y INVENToRs y v {,M. 'L

' l ATTORNEYS known to those in "thefart stop-bath may be eiective, the acidity thereof 'must be maintained, the advantages of which are obvious.

Patented Mar. 8, 1949 UNITED *STATES UFFICE STDB BATH ACIDITYJNDICATOR .'Harold D. Russell vand John'i-'L Crabtree,'Roch ester, N. Y., assignors toEastman'KodakCompany, Rochester, N. 'Y.,.: a\ oorporati0n:;,of New Jersey Application February 9, 1945,' Serial'No.` 577,104

6 Claims. 1

'The present invention relates to'an'in'dicator,

fand more particularly toan indicator'for visually indicating the strength or usefulness'of asolution 1such, for-example, as'the degree of` acidity of a photographic acidV 'stop-bath `for photographic prints'or acid xing'bath' for' iilms.

As is well known, photographic printsor negaltives, afterbeing developed, are ydrained briefly 'and then thoroughly rinsed in an acid stop'bath. .'Such a bath `serves to stop further action of the developing agents onthe: prints or negatives, and also reduces therriskvv of'stains; andin addifftion, lengthenszthe'useful life of the fixing bath by neutralizing-the alkalifof the developer carried over by the prints or negatives,y allaswell In l.order that the The present invention has, therefore, as its principal object :the vprovision 'vof v an indicator *which will visually designate the degree ofacidity of the stop or'xing bath and also 'the rchanges .in such acidity.

Another'object of the invention-isthe provision l of an indicator whichwill4 designate such acidity and the changes thereof Without the necessityfcvf ffchemical. tests.

A further objecty of thevinventicn'is the provi- ,l

vsion of such an indicator'which is 'suitablefor'l -use in a darkroom illuminatedybyrsaielights.

`Yet another object of the invention is theprovision of such'ran'indicator'lwhich*is simple-in construction, easy to use, accurate lin its results,

and very inexpensive to make. i

Tothese and otherzends, thefinvention resides :1in 'f certain ,improvements and; combinations of lparts, all as will-be 1hereinafter more fully.f de- -scribed, .the novel features vbeing pointed ,cutin '..the'claims atthe end oithegspecication.

-L'Inthe drawings:

.,Flg.. 1;,issa vvieweof thein'dicator .of the .present nvention, ushowing 'the relationV :of parts ,prior "to theirusein a stop orzxingbath and inthe Uconditionreceived by vthe purchaser or user;

Fig. 2isa view similar to'1ig.1,'but showing the 'relation f of Vthe 4parts when the indicator yis positioned in the bath Or-theproperacidity to `indicate suchA acidity;

'vfshowingA the relation" of parts -When the solution vonbath is vpartly neutralized; sand Fig.. i4 isaview'similar to the'other yviews but showing the relation offpartsr-when"the-solution 1lhasbeen A:exhausted to indicate :'r that the ba'th- 'and l4-in thedrawings. fthe `Word"Not is printed in black ink While is-no' longer' useful and should, thereforabeldiscarded or replenished.

'The 'same referencev numerals throughouty the -various views indicate thesame parts.

The'invention relates to an indicator for use When prepared for use in.: anf acid stop bath the paper Il is ldyed with asuitable 'dye' such, for example asfCongo redwhich colors the paper H a "bright red or crimson color.

This dyed paper is 'of a bright red color prior touse; but,when placed inl an-acid solution, such as an acid stop-bath, changes color to va yblue-black, but gradually Vchanges back tore'd again as the bath becomes neutralizedv or exhausted.

V"The paper H'has printed` thereon the words "-Notf "Very, and Aci'd: as shown at l2, I3, Foracid stop-baths,

the Word Very is printed in red ink, and the Work .Acid `is printed in vwhite ink. The inks used in printingthesewords may be of any rsuitable'type which are Vnot readily attacked Iby 'Ithefchemicals of `the stop-bath. The color of the word Very isf'slightlydarker than' the color of the'fCongoredz dye. 'All the words are, therefore, :clearly visibleuin'the new or unused inf dicatcr, as shown in'Fig.` l. 'When-however, the

paper Il is placedfin an acid'stop-bath ofthe proper acidity,'the paper' will turnblue-black in color. As-ftheiword"ffNot1isprinted in`black finkit will not nowrbe visible,'an'd only-the Words "'fVery acid=canr1be"seen under aSeries OO or "OA safelight'to 'thus clearlyA and visibly Vindicate vthat the'fbathisbf theproper acidity, as clearly ``illustrated Aiii-Fig. -2.

".Howeven `as thevstop-bath is used, it iWillbe -fgradually neutralizedfdue 'to 'the alkali of the developer. *Due to such neutralization, the paper I|"will gradually-turn lfrom the blue-black color illustrated `inliig. 2 v back *to* the original-red color, and the -word "NctL will-'gradually vbecome more and more visible, while the Word Very *will l'gradually become"invisible At -or'r near a certain f acidity both `Not" zand" Very will `both f-bevisible-fso that the paper will -read Not very acid to indicate that the stop-bath is not very acid, as illustrated in Fig. 3. Finally, when the bath has been neutralized to a point where it is no longer useful, the paper I l will return to its red color and the word Very will become substantially invisible under the safelight. The words Not and Acid will then be the only words visible to clearly indicate that the solution is not acid, as illustrated in Fig. 4.

Thus when the solution is very acid the words is of suiiicient acidity, but on partial neutralization the Words Not very acid are all visible to indicate that the solution is no longer at its original strength or acidity. Finally when the solution has been exhausted only the words Not acid are visible to indicate the lack of efficiency of acidity. Thus the paper will change color as the acidity of the bath varies, and the differently colored words serve to indicate not only the degree of acidity of the stop-bath, but also the changing of such acidity. v

When the bath has been completely neutralized the words Not acid appear, p and the bath should be discarded or renewed with a quantity of acid equal to that originally present in the bath. The test pap-er ll, in the renewed solution, will not be colored blue-black, as in the fresh solution but will indicate that the bath is partially exhausted. Furthermore, the life of the bath before the Words Not acidl appear will be approximately two-third-s that of the fresh solution. With use, the color of the paper gradually fades, and an old paper, which has been used several times, may not give exactly the same 4color reaction as that of a fresh paper. However, the paper may be used safely and reliably for about one week. The dry unused paper indicator should, however, be stored in an air-tight container in a darkroom as the paper will tend to bleach in sunlight and will change color if stored in a room in which there are acid vapors.

While the above indicator has been described primarily with relation to an acid stop-bath for prints, such an indicator is also adapted for -use in an acid fixing bath which functions in the same manner as a stop-bath, namely, it arrests development and prevents the formation of silver stains.

As a hardening fixing bath becomes neutralized, it gradually loses its hardening properties, and with baths containing aluminum and chrome alum, when the pI-I of the bath reaches a definite value, the bath tends to precipitate a sludge of aluminum or chromium hydroxide which may deposit as a scum on the film. VIt is important, therefore, to discard or revive an acid fixing bath when the pH reaches a definite limiting value.

l The Congo red indicator paper functions satisfactorily in certain fixing baths, specifically, a

Kodak F-5 fixing bath. When such a bath is fresh, the bath has a pH value of 1.0 and a value of 6.0 when exhausted at which time the indicator will designate Not acid. With another fixing bath, such as the Kodak F-6 lodorless xing bat-h, the Eastman X-Ray Fixer (powders), and the Kodak Liquid X-Ray Fixer, the fresh bath has a pH value of 4.6 which rises to 6.0 on exhaustion. With this latter bath it would be necessary to change the word Not to a reddish blue, which is the hue ofthe Congo red at pH of 4.6.

With chrome alum stop baths and chrome alum fixing baths, it will be necessary to change both ythe background dye and the inks. When fresh,

10 Very acid are visible to indicate the solution 4 the pH value of these baths is about 3.0 which rises to 4.0 on exhaustion. For use in such baths, it is preferred to have the paper ll dyed or colored with a brom phenol blue which is yellow at a pH of 3.0 and changes to blue at 4.0. The Words would then be colored as follows:

Not-Yellow Very-Blu'egreen Acid-Remaining white An alternative indicator dye is benzo purpurin which changes from violet at a pH of 3.0 to red at a pH of 4.0 so that the same color inks could be used as in the stop-bath arrangement, above described.

The above-described indicator thus provides a simple, inexpensive and readily readable means for indicating not only the actual acidity of an acid stop or fixing bath but also the changing of such acidity. Such an indicator may be placed in the tray holding the solution or may be attached to the tray by means of adhesive strip or clip but extending into the solution. As the color change of the paper requires about one minute, the paper indicator is not recommended for `dipping tests but only for tests in which the indicator remains in the solution for a substantial time.

While one embodiment of the invention has been disclosed, it is to be understood that the invention may be carried out in a number of ways. This application is not, therefore, to be limited to the precise details described, but is intended to cover all variations and modifications thereof falling within the scope of the appended claims.

We claim:

1. An acidity indicator comprising, in combination, a paper sheet dyed with a Congo red dye so that the sheet will have a dark color when placed in an acid solution but will gradually turn back to red as the solution becomes neutralized, three indicating symbols printed on said dyed sheet, each symbol being printed in a different color ink so that When the sheet is in acid solution, only two of said symbols will be Visible to indicate that the solution is acid, but as the solution becomes neutralized, one of said two symbols will become substantially invisible in a weak illumination while a third symbol will become visible and cooperate With the other of said two symbol-s to indicate that the solution has been neutralized.

2. An acidity indicator comprising, in combination, a paper sheet dyed With a Congo red dye so that when in an acid solution the sheet will be of a dark color but will gradually turn to red as the solution becomes neutralized, a word Not printed with black ink on said dyed sheet, a Word Very printed with red ink on the dyed sheet, and a word Acid printed in white ink on said sheet, said inks being -stable and not attacked by said solutions, the Words Very and Acid being visible when the sheet is lin an acid solution to indicate visually thev acidity but the word Very becomingsubstantially invisible in a weak illumination vand the word Not becoming visible and cooperating with the word Acid when the solution has become neutralized to indicate that the solution is now neutralized.

3. An acidity indicator comprising a paper sheet impregnated with an indicator which is red in a neutral solution and black in an acid solu tion, and superimposed markings thereon, one of said markings being of such red coloration to blend with the color of the sheet When moistened with neutral solution and contrasting-with the black coloration of the sheet when the sheet is moistened with acid.

4. An acidity indicator compri-sing a paper sheet impregnated with an indicator which is red in a neutral solution and black in an acid solution, and superimposed markings thereon, certain of said markings being of such colorations to contrast with the black coloration of the sheet when the sheet is moistened with an acid solution but one of said certain markings being of such red coloration as to blend with the red coloration of the sheet when the latter is moistened with a neutral solution, and another superimposed marking being of such black coloration to contrast with the red coloration of the sheet when the sheet is moistened with a neutral solution but blending With the black coloration of the sheet when the sheet is moistened with an acid solution.

5. An acidity indicator comprising a paper sheet impregnated with an indicator which is red in a neutral solution and black in an acid solution, and three superimposed color markings thereon, one of said markings being of a White coloration so as to contrast with the red coloration of the sheet when the sheet is moistened with a neutral solution and also to contrast with the black coloration of the sheet when moistened with an acid solution, a second marking of such red coloration to contrast with the black coloration of the sheet when the sheet is moistened with an acid solution and co-operating with said white marking to indicate the acidity of the solution, said red marking blending With the red coloration of the sheet when the sheet is mois- 5 Number tened with a neutral solution, and a third marking of such black coloration as to blend with the black coloration of the sheet when moistened with an acid solution but to contrast with the red coloration of the sheet when moistened with a neutral solution and co-operating with said white marking to indicate a neutral solution.

6. An acidity indicator comprising a paper sheet impregnated with an indicator which is one color in a neutral solution and a different and contrasting color in an acid solution, superimposed markings thereon, one of said markings being of such coloration to blend with the color of the sheet when moistened with a neutral solution and contrasting with the color of the sheet when the sheet is moistened with acid.

HAROLD D. RUSSELL. JOHN I. CRABTREE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Dieterich Jan. 14, 1902 Kloz July 25, 1939 Snelling July 22, 1941 McClintock Mar. 23, 1943 FOREIGN PATENTS Country Date France July 4, 1913 Germany June 22, 1933 Number 

